FIG. 1 schematically illustrates an optical computer mouse 1, comprising a laser device 2. The laser device is implemented as a semiconductor laser, as known per se. The mouse is moved over a surface 4, for instance a desktop. From a power source not shown for sake of simplicity, the laser device 2 receives an electric current, and as a result the laser 2 emits a laser beam 3 with a certain wavelength, which is reflected by the surface 4. Part of the laser light is reflected back towards the laser. It is possible to derive, from this reflected laser light, a signal representing velocity of the movement of the mouse 1 with respect to the surface.
FIG. 2 is a schematic diagram illustrating the measuring principle. The laser 2 comprises a semi-transparent front mirror 11 and a semi-transparent rear mirror 12, with a laser medium 13 (semiconductor body) between said two mirrors. It is noted that the mirrors 11, 12 are shown as two-dimensional structures, but in practice the mirrors 11, 12 will have a layered structure.
The laser light within the laser medium 13 is indicated as main laser light L0. Part of the laser light passes the front mirror 11 and forms the output beam 3; this light is also indicated L1. Likewise, part of the laser light passes the rear mirror 12 and forms a measuring beam 5; this light is also indicated L2.
The object 4 can be considered to constitute an external mirror with diffuse properties, and reflects the incoming beam L1: this is indicated as a reflected beam L3. In the drawing, the reflected beam L3 is shown as being a one-dimensional beam making an angle with the incoming beam L1, but in practice the reflected beam L3 will have a certain spatial distribution and a portion of this reflected beam L3 will be directed towards the front mirror 11. Thus, the object 4 can be considered as defining an external cavity together with the front mirror 11.
Under stationary conditions, the light L0 within the laser medium 13 forms a standing wave. Likewise, light L1 and L3 in the external cavity forms a standing wave which, through the front mirror 11, interferes with the light L0 within the laser medium 13. The measuring beam 5 has a constant intensity.
Assume that the object 4 is moving away from the laser 2. This means that the length of the interference cavity between the front mirror 11 and the object 4 is increasing, i.e. the number of standing waves fitting between the front mirror 11 and the object 4 is increasing. Consequently, the interference state at the location of the front mirror 11 changes from fully constructive to fully destructive and back. This has influence on the interference state in the laser medium 13, which in turn has influence on the intensity of light L5 of the measuring beam 5. As a result, this light L5 has intensity fluctuations at a frequency fD that is proportional to the velocity of movement of the object 4 with respect to the laser 2, i.e. the component thereof along the optical axis. It should be clear that the measuring beam 5 can be detected by an optical sensor, and that its output signal can be processed by a signal processor in order to process these intensity fluctuations and to calculate the object velocity therefrom. It is noted that said frequency fD is equal to the Doppler frequency.
A problem in this respect is that the same explanation applies, irrespective of the object moving towards or away from the optical detector. In other words, with the simple measuring build-up as described above it is impossible to determine movement direction.
It has already been proposed to solve this problem by supplying the laser with a triangularly modulated laser current, as illustrated in FIG. 3A. The laser current is varied in a linear manner between two extreme values I1 and I2 having the same sign. During one half of a current period, the laser current I is increasing from I1 to I2, the change rate R1=dI/dt being substantially constant. During another half of a current period, the laser current I is decreasing from I2 to I1, the change rate R2=dI/dt being substantially constant; typically, R2=−R1. Increasing/decreasing the laser current causes an increase/decrease of the laser temperature (as illustrated in FIG. 3B), which in turn causes an increase/decrease of the wavelength of the laser light (as illustrated in FIG. 3C) with a substantially constant change rate dλ/dt, in which λ indicates the laser wavelength. The result can be explained as follows. Assume that the object is moving away from the laser, so that the length of the interference cavity between the front mirror 11 and the object 4 is increasing. If the current magnitude and hence the laser wavelength is also increasing, the frequency of the intensity fluctuations of measuring light L5 is reduced; this is illustrated by a peak f1 in the frequency spectrum of FIG. 3D. The reduced frequency may even become equal to zero if D/λ remains constant, in which D indicates the distance between the front mirror 11 and the object 4. Conversely, if the laser wavelength is decreasing, the frequency of the intensity fluctuations of measuring light L5 is increased; this is illustrated by a peak f2 in the frequency spectrum of FIG. 3B. It is noted that the shift |fD−f1| is equal to the shift |fD−f2|. The spectrum of the intensity fluctuations of measuring light L5 thus shows two peaks f1 and f2, as schematically illustrated in FIG. 3B. If on the other hand the object is moving towards the laser, a frequency spectrum with two frequency peaks is again obtained, but now the lower frequency is obtained during the periods that the current magnitude is decreasing. It should be clear that this information can be derived from the measuring signal relatively easily by a suitably programmed signal processor.
For a more detailed explanation, reference is made to U.S. Pat. No. 7,339,683, the contents of which is incorporated here by reference.